A typical method of operation for laser detection and ranging (LADAR) is to emit a short (˜ns) optical pulse in a narrow beam and detect its reflected return from a target. The time delay between emission and detection provides the range to the target, and knowledge of the transmitted beam angle/direction provides target bearing information. The transmitted beam is collimated to provide low divergence to the target. In general, the target will reflectively scatter the incident beam into a large solid angle, so the amount of reflected power detected at the LADAR system will be proportional to the area of the receiver aperture. If the transmitted and received beams share a common aperture the system is described as monostatic. This type of system is insensitive to the misalignment and parallax problems common to LADARs which have separate transmit and receive apertures (described as bistatic). In monostatic operation, an optical means is provided for separating the transmitted and received beams before the aperture, so that the transmitted beam does not lose energy or degrade the operation of the photodetector, and the received intensity is directed primarily to the photodetector. In principle, this optical diplexing function might be as simple as a beam-splitter (which has excessive round-trip optical loss or more complex (such as a non-reciprocal optical circulator). In both cases (as in the bistatic system), careful alignment of the optical components must be performed during the assembly to assure that the narrow FOV of the small photo detector used as the high bandwidth receiver is aligned (in angle) with the transmitted beam.
To gather three-dimensional imagery for such a narrow-beam, single-detector system, the transmit/receive (TX/RX) direction must be moved or scanned over the desired field-of-view. This task is typically achieved using mechanical scanners such as rotating polygon mirrors, galvanometer scanner mirrors, or piezo-electrically scanned mirrors. These scanning techniques suffer from high cost, large volume and weight, and/or limited scanning speed and angle. An alternate method for implementing a LADAR system is to flood the entire field of view (FOV) with the transmitted beam, and simultaneously detect the reflections over many pixels imaging that FOV. This mode of operation is referred to as “flash” LADAR, and requires higher energy laser pulses with a complex camera focal plane photo-detector array and readout circuit.